This invention relates to the field of video sync signals for CRT display devices and more particularly to the provision of an improved digital vertical synchronization arrangement.
A CRT display device conventionally requires horizontal and vertical sweep signals in order to form a raster for the fields of the display. These sweep signals are produced by saw-toothed generators in the display device which are caused to sync with the synchronizing generator of the original signal by means of composite sync information included in the received signal above the black level. The composite sync information can be separated from the picture information by amplitude clipping. The composite sync signal is then separated into its horizontal and vertical components. The vertical component is processed in any of various ways to produce a control or trigger pulse for a free-running oscillator in the vertical sweep circuit which requires a hold control to keep it sufficiently close to the vertical sync frequency for synchronization to occur. When a weak or noisy video signal is received, the output of a typical sync separator may not include a vertical sync signal strong enough to sync the saw-tooth generator in the vertical sweep circuit and thereby prevent "rolling" of the display. Another approach would be to have a "one-shot" ocsillator triggered by each input sync pulse, but, in this case, if the input disappears, the vertical sweep would cease. The resultant horizontal line could damage the fluorescent screen of the display device.
Many methods of dealing with these twin problems have been devised, but did not completely eliminate the need for a hold control in the vertical sweep circuit.